Types of Nuclear Radiation:

There are three types of nuclear radiation which may be detected with a geiger
counter:

Alpha Particles: Helium nuclei, generally emitted from heavy
elements such as uranium and thorium. Alpha particles only travel a few
inches in the air, and can be stopped by a piece of paper. Special geiger
tubes with a mica window are necessary to detect them, as other windows
will stop alpha particles.

Beta Rays: Electrons moving at extremely high (often
relativistic) speeds. They are more penatrating than alpha particles.
They can pass through light elements, such as paper and aluminum (but only
small thicknesses).

Gamma Rays: Electromagnetic waves, similar to light, but at a
much higher energy. Much more penetrating than alpha or beta radiations.
High energy gamma rays can pass through several inches of metal. Note that
X-Rays and Gamma Rays are really the same thing, the term X-Ray is used
when the radiation is produced by electrons striking a material, such as
in an X-Ray tube.

Typical Geiger Counter Circuit:

When a particle enters the tube, the gas is ionized, and can conduct
current from the 500V DC supply. The 10M resistor limits the current to a
safe level. A quenching gas (typically a halogen) stops the flow of
current a few microseconds later. Thus, a pulse of current flows. This
pulse is passed by the capacitor, which blocks the 500V DC. The output
pulse can then go to an amplifier if necessary, and then some sort of
counter.

Typically, the counts are summed over a one minute period, and therefore
the common unit is Counts Per Minute (CPM). The number of CPM depends on
the size and efficiency of the GM Tube. A larger tube naturally will
detect more particles.

I've built a circuit which powers the GM tube strictly from the serial
port handshake lines - no other power supply is necessary. The circuit
then conditions the detected pulse and sends it back to the computer,
where software can tally the number of pulses detected over a time period,
typically CPM - Counts Per Minute.

Sources of Radiation:

There are many
Sources of Natural Radiation,
radiation is all around us, naturally.
Radon gas exists
in most parts of
the US, in varying levels depending upon where you live. Radon is produced
from naturally occuring Uranium-238 in the soil. Radon is a problem in some
areas today because homes are much more air-tight than they used to
be. The radon gas enters the house through the basement. Thorium-232 also
exists in the soil. Uranium and Thorium decay into numerous other
radioactive isotopes before finally decaying into a stable element such as
lead. And all this occurs naturally. In fact, the decay of uranium and thorium is
the principle source of energy the heats the center of the Earth.
Radiation existed long before Man,
even though some would have you believe otherwise.

The US Geological Survey has a Radon WWW
Page. It contains a great deal of useful information, maps of radon
levels in the US, and links to other sites.

Note that even though most of the heavy elements are alpha emitters, they
can still be dangerous, if they get inside your body. Your skin stops
alpha particles if the source is outside your body, but your internal
organs and tissues have no such protection. Radioactive dust can be
hazardous. Radioactive dust? Read on...

High Altitude Radiation from Airplane Flights

When you fly in an airplane, you're up above much of the Earth's atmosphere. It's the air
that protects us from a lot of radiation due to cosmic rays coming from outer space.

Recently I took a transcontinental flight, and of course a
GM-10
Radiation Detector
came along for the ride!

The graph below shows the background radiation levels on the ground, as well as when we
were at our crusing altitude (around 35,000 feet). Note the huge difference in radiation
levels! The CPM (Counts Per Minute) went from about 12 on the ground to 360 in flight, or 30 times the level!

Radioactive Dust

Did you know that the dust that's in the air and settling all over your
house (and computer monitor) is radioactive? It's true, it contains
radioactive decay products from naturally occuring Uranium and Thorium.

As an experiment, I wiped some dust from the TV screen onto a tissue, and
placed it in front of the radiation detector. The reading went from a background
reading around 10 CPM to around 1300 CPM, or 130 times the reading!

This graph shows the radiation (in Counts Per Minute, CPM) over time, as the daughter products
decay. In addition to plotting the raw data, I've also estimated the initial amounts of
Radium B (Pb214), Radium C (Bi214) and Pb212:

Radium B and Radium C are decay products of Radon. The Radon is produced
by Uranium that naturally occurs in the soil, after a rather long decay
process. Radon has a half life of just under 4 days. In addition to being
produced by local sources, Radon (and it's daughter products) can be blown
in by the winds from distant locations. So just because you don't have a
radon problem in your basement doesn't mean that you won't find
radioactive dust in your house!

The Radon decays into
Polonium, which then decays into Radium B, an isotope of Lead. This decays
with a half life of about 27 minutes in Radium C, an isotope Bismuth. This
decays with a half life of about 20 minutes into another Polonium isotope,
which quickly (164 microseconds) decays into an isotope of Lead. The Lead
decays with a very long half life of 22 years, so we don't get much
radiation from it, or any further products.

The two Radium isotopes both undergo beta decay, and it is their radiation
we detect in this experiment. Notice that we initially start with much
more Radium B than Radium C. The Radium B decays, producing more Radium C,
initially at a rate faster than the Radium C is decaying. So The amount of
Radium C starts to increase. Eventually, there is not enough Radium B
decaying into Radium C, and the amount of Radium C starts to drop.

Here's the process:

Radon (Rn222) does an alpha decay into Polonium
(Po218) with a half life of 3.824 days.

Polonium (Po218) does an alpha decay into Lead
(Pb214) with a half life of 3.05 minutes.

Lead (Pb214) does a beta decay into Bismuth
(Bi214) with a half life of 26.8 minutes.

Bismuth (Bi214) does a beta decay into Polonium
(Po214) with a half life of 19.8 minutes

Polonium (Po214) does an alpha decay into Lead
(Pb210) with a half life of 164 microseconds.

Lead (Pb210) does a beta decay into Bismuth
(Bi210) with a half life of 22.3 years.

Bismuth (Bi210) does a beta decay into Polonium
(Po210) with a half life of 5.01 days.

Polonium (Po210) does an alpha decay into Lead
(Pb206) with a half life of 138.38 days.

Lead (Pb206) is stable.

The black line at the bottom of the graph shows the amount of Lead-212
present. This is a daughter product of Thoron, an isotope of Radon which
is produced by Thorium, rather than Uranium. Thoron has a very short half
life, about 1 minute. Lead-212 has a much longer half life, almost 11
hours.

I was doing laundry one day, and decided to check out the lint that collects in the
lint filter in the clothes dryer. It produced counts around 240 CPM, again much higher than the
background readings. This is probably due to the numerous uranium/thorium atoms and
daughter products that are floating in the air, which get stuck to your clothes, and are sucked
into the dryer.

Many parts of the US, including Maryland, where I live, have relatively
high amounts of radon gas. This was never a problem in the past, but many
modern homes are very air-tight, to reduce energy costs. This means that
the radon gas is more likely to be trapped in the house, where it can
decay into the much more dangerous daughter products.

Radon is much more of a problem for smokers. The radiation dust attaches
itself to the smoke particles, and they get trapped to the sticky goo that
gets stuck inside your lungs, bombarding your tissue with alpha radiation.
(As if smoking was bad enough for you)

Radon Test Kits:

There are low cost ($15) test kits you can buy. You place a small cylinder
(containing charcoal) in your basement. Radon daughter products deposit
themselves in the container. You record the exact start and stop times for
the collection period (typically a few days) and then mail the canister
into a lab for analysis (lab fees built into the price). They use
sensitive scintillator/PMT detectors to measure the amount of radiation,
then adjust the results based on the collection period, and how long it
took the canister to get to them in the mail. They then send you a report
indicating the estimated radon level in the area, expressed as picocuries
per liter of air.

A curie is a measure of the activity of radiation. It is equal to 37
billion decays per second. The metric equivilent is the becquerel, which
is one decay per second. So a picocurie is 0.037 decays per second. Currently
the EPA considers 4 picocuries to be the safe limit. This would be 0.148
radon decays per second. Radon has a half life of about 3.83 days, this
equates to around 100 thousand radon atoms per liter of air. Again, radon
itself is not very dangerous, the risk comes from the radioactive daughter
products (dust) that can get inside your body.